1. Field of the Invention
The present invention relates to a metallic gasket which is interposed between joining surfaces of a cylinder block and a cylinder head of an internal combustion engine, and which prevents leakage of fluid such as combustion gas, cooling water, lubricating oil, and the like.
2. Description of the Prior Art
Generally, a gasket of various kinds is interposed between joining surfaces of a cylinder block and a cylinder head mounted thereon of an automobile engine such as an internal combustion engine to prevent leakage of fluid such as combustion gas, cooling water, lubricating oil, and the like, and the sealing function is achieved by tightening both the cylinder block and the cylinder head by tightening members such as bolts or the like. Conventionally, as such a gasket, the so-called soft gasket is provided which is formed of an asbestos material, a chemical fiber material, a carbon sheet material, or the like. However, the soft gasket involves a drawback as an inavoidable problem due its material that there is a deterioration with time due the operation of an internal combustion engine, and that it is difficult to ensure a heat resistance. As a result, is well known that recently, metallic gaskets are widely used, which are excellent in heat resistance, compressive resistance, and durability, and which provide satisfactory restoration properties (spring characteristics) and thermal conductivity.
In such metallic gaskets, the sealing in a peripheral portion of the combustion chamber, that is, the cylinder bore is particularly important. In other words, when the sealing of this portion is not sufficient, the utilization of the combustion gas in the inside of the combustion chamber is incomplete, and it will result in a reduction of the pressure. Accordingly, an elastic base plate in the metallic gasket is provided with a bead formed concentrically with a cylinder bore hole which is formed to communicate with the cylinder bore, and the surface pressure generated between the joining surfaces of the metallic gasket and the cylinder block, and of the metallic gasket and the cylinder head is made to increase by utilizing a repulsive force which is generated when the metallic gasket is tightened by the tightening members such as the bolts between the cylinder block and the cylinder head, thereby to improve the sealing in the peripheral portion of the cylinder bore.
The bead is elastically deformed in a direction in which the bead collapses by the surface pressure when the metallic gasket is tightened by the tightening members such as the bolts between the cylinder block and the cylinder head, however, in order to completely collapse or flatten the bead, a relatively large load is needed. However, between the joining surfaces between the cylinder block and the cylinder head, the tightening force is large at a position near the tightening member such as the bolt, however, the tightening force is inevitably decreased at a remote position. This is resulted from the fact that when an aluminum alloy having low rigidity is used as a material of the cylinder head to decrease the weight of the engine, or when a width of the cylinder head between the combustion chambers is reduced by reducing a distance between combustion chambers to make the engine small in size and light in weight, the rigidity of a portion between the combustion chambers of the engine is reduced.
On the other hand, the bead is formed at a position remote from the position of the tightening member such as the bolt. Accordingly, due to the distance from the tightening member such as the bolt and the above-mentioned reduction of the rigidity of the engine, the bead does not completely collapse by the surface pressure, and an inclination is caused in the bead. More specifically, at a portion at which the bead is formed, the rigidity of the engine and the repulsive force of the bead are balanced, and this balance is maintained when the engine is stopping. However, when the engine is operating, due to an increase and a decrease of the pressure inside the combustion chamber, the tightening force imparted to the metallic gasket is changed, and vibrations are generated in the bead. Furthermore, as a result of vibrations due to a mass of the engine, and as a result of a change in volume due to a temperature change of each engine component, variations are caused in the tightening force imparted to the metallic gasket. Thus, the bead is vibrated by its elasticity due to the above-mentioned vibrations and the variations in the tightening force, and furthermore, there is a possibility of causing a fatigue break when the bending of the bead is repeated due to the vibrations.
Accordingly, there is a proposal to restrict the magnitude of an amplitude of the above-mentioned vibrations and to prevent the occurrence of the fatigue break at the portion of the bead, as disclosed, for example, in Japanese Utility-Model Publication Hei No. 4-16026, and Japanese Patent Publication Hei No. 5-61503.
Specifically, in Japanese Utility-Model Publication Hei No. 4-16026, as shown in FIG. 10, a subplate 3 is placed over a base plate 1 on a side to which a bead 2 expands, that is, the side of a convex of the bead 2, and the base plate 1 and the subplate 3 are coupled with each other by spot welding or by using eylets at several positions including these portions (for example, 10a in FIG. 9 which will be described later) which extend outwardly beyond a joining area with the cylinder block and the cylinder head of the engine. Furthermore, an edge of the subplate 3 is folded back to the base plate 1 side to form a folded section 3a, and this folded section 3a is positioned at an inner side towards the combustion chamber with respect to a starting point or turning point 2a of the bead 2, which starting point 2a is located near the combustion chamber, so that the folded section 3a and the starting point 2a do not overlap with each other. In such a metallic gasket, by the formation of the folded section 3a, the complete collapse of the bead 2 in a collapsing direction is prevented, and at the same time, the deterioration of the restoration property (spring characteristic) is prevented, thereby to decrease the amplitude of the vibrations and to prevent the occurrence of the fatigue break.
On the other hand, in Japanese Patent Publication Hei No. 5-61503, as shown in FIG. 11, a subplate 3' is placed over a base plate 1 at a side at which a bead 2 does not expand, that is, at a concave side of the bead 2, and the base plate 1 and the subplate 3' are coupled with each other at several positions including these portions which extend outwardly beyond a joining area with the cylinder block and the cylinder head in a similar manner as described with reference to FIG. 10. Furthermore, an edge of the subplate 3' is folded back upwardly to hold a flat portion la of the base plate 1 by the folded edge portion of the subplate 3', thereby to form laminations. In this structure, at the laminated end portion, a thickness is increased by one sheet of plate thickness of the subplate 3' as compared with other part of the base plate 1. As a result, a high surface pressure can be ensured at this laminated end portion. Furthermore, by virtue of the structure, a high pressure gas generated in the combustion chamber during operation of the engine is initially sealed (primary sealing) at the laminated end portion, and further, the high pressure gas is secondary sealed by collapsing the bead 2 in the collapsing direction. Accordingly, even when the tightening force of the metallic gasket is varied and the vibrations of the bead are caused, the deformation of the bead 2 in the collapsing direction is restricted by the end portion which is folded, and the vibrations are reduced and the occurrence of the fatigue break is prevented.
However, the metallic gaskets in the prior art involve the following drawbacks.
Specifically, in the metallic gasket in Japanese Utility Model Publication Hei No. 4-16026, the base plate 1 and the subplate 3 are coupled with each other only at several positions at the portions extended outwardly beyond the joining area with the cylinder block and the cylinder head. Accordingly, in the case where the subplate 3 is thin, for example, when the cylinder block of the engine is made of cast iron and the cylinder head is made of aluminum, and when the engine is operated and stopped to repeat a temperature rise and fall in the combustion chamber, there is a possibility that the subplate 3 which is thiner than the base plate 1 is deformed or produces a wrinkle, so that the subplate 3 protrudes into the combustion chamber, or a crack is caused in the folded section 3a. On the other hand, when the subplate 3 is thick, a folded end of the subplate 3 does not become completely flat, or the folded end will somewhat expand to both sides towards the joining surfaces. As a result, the folded end is difficult to be completely flattened even by the tightening force of the tightening members such as bolts, and the folded end will be subjected to a large load at its upper and lower ends in the thickness direction of the subplate 3. Accordingly, a sealing surface of the cylinder block or the cylinder head will sink or become hollow at a peripheral portion of the combustion chamber, and on the other hand, when an enormous load is applied to completely flatten the folded end, there is a possibility that the folded end is broken.
Also, in the metallic gasket in Japanese Patent Publication Hei No. 5-61503, since the base plate 1 and the subplate 3' are coupled with each other only at several positions outside the joining surface area, and since the folded section 3a' is directly in contact with the joining surface of the cylinder block or the cylinder head at the time of assembling the metallic gasket to the engine, when the temperature rise and the temperature fall in the combustion chamber are repeated due to the operation and stopping of the engine as described in the foregoing, the degree of thermal expansion will be different when the engine components are made of different kind of materials, there is a possibility that the subplate 3' which is thiner than the base plate 1 is deformed, and a wrinkle is caused to extend into the combustion chamber, and a crack is caused in the folded section 3'.